1. Field of Invention
The present invention relates to an art for suppressing influence of an interference signal on a desired signal transmitted from a transmitter to a receiver.
2. Description of the Related Art
Conventionally, a wireless communication system such as a wireless LAN (Local Area Network) system and a digital cellular communication system includes a plurality of wireless communication apparatuses (that function as transmitters and receivers). These wireless communication apparatuses share a predetermined frequency band to perform communications with each other. Accordingly, a signal received by each of the wireless communication apparatuses includes a signal transmitted to the wireless communication apparatus (hereinafter referred to as a “desired signal”) and a signal that is irrelevant to communication of the wireless communication apparatus. The irrelevant signal is for communication between other wireless communication apparatuses. Note that a signal included in a received signal, other than a desired signal, is hereinafter referred to as an “interference signal”. The interference signal includes radio waves leaked from various apparatuses other than the wireless communication apparatuses included in the wireless communication system.
Next, the outline of a wireless communication system is described.
FIG. 28 shows a conventional wireless communication system. In FIG. 28, wireless communication apparatuses 1011 and 1012 perform communications with each other. Also, wireless communication apparatuses 1021 and 1022 perform communications with each other. In FIG. 28, as well as FIGS. 29, 30A and 30B, solid lines represent desired signals, and dotted lines and dashed-dotted lines represent interference signals.
For example, the wireless communication apparatus 1011 transmits a data packet D11 generated by packetizing transmission data to the wireless communication apparatus 1012. When normally receiving the data packet D11, the wireless communication apparatus 1012 transmits an ACK (ACKnowledgement) packet A12 to the wireless communication apparatus 1011.
Also, the wireless communication apparatus 1021 transmits a data packet D21 generated by packetizing transmission data to the wireless communication apparatus 1022. When normally receiving the data packet D21, the wireless communication apparatus 1022 transmits an ACK packet A22 to the wireless communication apparatus 1021.
There is a case where the data packet D21 and the ACK packet A22 transmitted and received between the wireless communication apparatuses 1021 and 1022 can be received by the wireless communication apparatuses 1011 and 1012, as an interference signal.
FIG. 29 shows an example of a transmission sequence of the wireless communication system shown in FIG. 28. In FIG. 29 and other figures showing transmission sequences, each of the wireless communication apparatuses is simply described as a “communication apparatus”.
For a period from a time T2 to a time T3, the wireless communication apparatus 1011 transmits the data packet D11 to the wireless communication apparatus 1012. For a period from a time T4 to a time T6, the wireless communication apparatus 1012 transmits the ACK packet A12 to the wireless communication apparatus 1011.
For a period from a time T1 to a time T5, the wireless communication apparatus 1021 transmits the data packet D21 to the wireless communication apparatus 1022. For a period from a time T7 to a time T8, the wireless communication apparatus 1022 transmits the ACK packet A22 to the wireless communication apparatus 1021.
For a period included in the period (from the time T2 to the time T3) for which the wireless communication apparatus 1012 receives the data packet D11, the wireless communication apparatus 1012 receives the data packet D21 as an interference signal. Accordingly, when the wireless communication apparatus 1012 is located adjacent to the wireless communication apparatus 1021 for example, a power of the data packet D21 received by the wireless communication apparatus 1012 is high. This increases the possibility that an error will occur in demodulation processing of the data packet D11. Likewise, when the wireless communication apparatus 1011 receives the ACK packet A12 concurrently with an interference signal, there is a high possibility that an error will occur in demodulation processing of the ACK packet A12.
Note that influence of an interference signal on a desired signal also depends on a channel frequency of the desired signal and a channel frequency of the interference signal.
FIG. 30A is a pattern diagram showing the CCI (Co-Channel Interference). When a channel band for a desired signal is the same as a channel band for an interference signal, the interference signal exerts a great influence on the desired signal. Asa result, there is a high possibility that an error will occur in demodulation processing of the desired signal.
FIG. 30B is a pattern diagram showing the ACI (Adjacent Channel Interference). When a channel band for a desired signal is different from a channel band for an interference signal, the interference signal exerts a less influence on the desired signal, compared with a case where the channel band for the desired signal is the same as the channel band for the interference signal. However, when an electric power leaked to outside the channel band is high, for example when the interference signal is a broadband signal and nonlinear distortion occurs in a transmission power amplifier, an error might occur in demodulation processing of the desired signal due to the influence of the interference signal.
Also, when the wireless communication apparatus 1011 coexists with apparatuses included in another wireless communication system and apparatuses that leak radio waves, such as apparatuses that radiate a 5 GHz band radar, a radio wave radiated by such apparatuses might exert influence on a desired signal, as an interference signal. This also might cause occurrence of error in demodulation processing of the desired signal due to the influence of the interference signal. Note that the wireless communication system using the 2.4 GHz band includes the wireless LAN system, the Bluetooth system, and the cordless telephone system. The wireless communication system using the 5 GHz band includes the wireless LAN system and the wireless access system. Also, the apparatuses that leak radio waves include microwave ovens using the 2.4 GHz band, for example.
In view of the above, as one of conventional arts for suppressing influence of an interference signal on a desired signal, an art handles a case where the interference signal is a radar periodically arriving at a wireless communication apparatus (See Patent Document 1, for example).
According to the above conventional art, the wireless communication apparatus determines a length of a packet to be output therefrom so as to be no more than a length of a transmission interval of interference signals that periodically arrive at the wireless communication apparatus. Then, the wireless communication apparatus transmits the packet while the wireless communication apparatus is not receiving the interference signal. In this way, the wireless communication apparatus performs control such that the packet to be transmitted therefrom (a desired signal) is not superimposed on the radar (an interference signal).    [Patent Document 1] Japanese Laid-Open Patent Application Publication No. 2001-257682.